Make MediaFileUtil run on thread pool.

webkit/fileapi/file_system_operation_context.h

Index: webkit/fileapi/file_system_operation_context.h
diff --git a/webkit/fileapi/file_system_operation_context.h b/webkit/fileapi/file_system_operation_context.h
index 7b265391ed242b5a575c8bc0e3a57b3fe48377b6..3cc48857503f7ef570550c31cec24a7cd96ad03e 100644
--- a/webkit/fileapi/file_system_operation_context.h
+++ b/webkit/fileapi/file_system_operation_context.h
@@ -35,7 +35,11 @@ class FILEAPI_EXPORT_PRIVATE FileSystemOperationContext {
   }
   int64 allowed_bytes_growth() const { return allowed_bytes_growth_; }
 
-  base::SequencedTaskRunner* file_task_runner() const;
+  base::SequencedTaskRunner* task_runner() const {
+    return task_runner_.get();
+  }
+
+  void SetTaskRunner(base::SequencedTaskRunner* task_runner);
 
   void set_media_path_filter(MediaPathFilter* media_path_filter) {
     media_path_filter_ = media_path_filter;
@@ -47,6 +51,7 @@ class FILEAPI_EXPORT_PRIVATE FileSystemOperationContext {
 
  private:
   scoped_refptr<FileSystemContext> file_system_context_;
+  scoped_refptr<base::SequencedTaskRunner> task_runner_;
 
   int64 allowed_bytes_growth_;
   MediaPathFilter* media_path_filter_;